The present invention relates to cooling systems for cooling electronic devices, and in particular integrated circuit devices.
Advances in integrated circuit chip technology toward a scaling down of the dimensions of the devices formed on the integrated circuit chip are resulting in increased chip size and complexity. VLSI's (very large-scale integrated circuits) use closely spaced interconnection lines to connect a larger number of scaled down devices in a smaller cross-sectional area than conventional integrated circuits. The increased number of devices operating in a smaller area results in increased heat generation, and reduced heat dissipation within the chip. The combination of increased heat generation and reduced heat dissipation can result in high operating temperatures that reduce the operating reliability of the chip, particularly when the chip is operated at high frequencies. Consequently, the operating frequency of the chip, which reflects the operating speed of the circuit, is limited by the heat generated in the circuit. Also, heat generated in relatively large integrated circuits can result in thermal degradation of the devices in the chip and of surrounding electronic components. Thus, it is desirable to remove the heat generated in the integrated circuit chips to obtain more reliable circuitry at higher operating frequencies.
Traditionally, heat sinks mounted on the chip in combination with fans that direct air flow across the heat sinks, are used to cool the integrated circuit chip. Conventional heat sinks comprise metal fin structures mounted on the packaging of the integrated circuit chip to dissipate the heat generated by the integrated circuit over a large surface area. Fans situated proximate to the metal fins direct air through the fins. However, the metal fins of the heat sinks are limited to sizes suitable for mounting on the integrated circuit chip packages. Because the integrated circuit chip packages have small areas, the metal fin structures are limited in size and do not effectively dissipate the heat generated by the integrated circuit chips. Also, it is often difficult to position the fans close to the integrated circuit chips within housing enclosures containing tightly packed electrical circuitry to achieve a uniform air flow across the metal fins. Thus, integrated circuit chip package temperatures of up to 70.degree. to 100.degree. C. are often prevalent during high speed operation of the chips.
Thermoelectric coolers have also been used to cool the integrated circuit chips. Thermoelectric coolers comprise dissimilar semiconductor elements that are electrically connected at a hot junction and a cold junction. Heat is absorbed at the cold junction at a rate proportional to the number of semiconductor elements and amount of current passed through the elements. One example of a thermoelectric cooled integrated circuit package, disclosed in U.S. Pat. No. 5,032,897, consists of an multilayer ceramic, electrically insulative, thermally conductive, chip carrier module that defines a cavity. A thermoelectric cooler is positioned in the cavity in contact with an integrated circuit, and conductors passing through channels in the module connect the integrated circuit to the external environment.
However, there are several problems with existing thermoelectric cooled packages for integrated circuit chips. One problem is that the heat generated at the hot junction of the thermoelectric cooler during operation of the thermoelectric cooler, increases the temperature within the housing enclosure surrounding the chip. This causes adjacent electrical circuitry and integrated circuit chips to heat up thereby reducing their reliability. Using a fan to cool the thermoelectric cooler only serves to circulate the additional heat generated by the thermoelectric cooler within the housing enclosure. Another problem occurring when the cold junction of the thermoelectric is operated at lower than ambient temperatures, is that condensation of ambient moisture results in dew or frost formation on the integrated circuit device and on the surrounding electrical circuitry. The condensed moisture can cause shorting and electrical failure of the surrounding electrical circuitry. Solutions relating to the use of non-standard custom packages for the integrated circuit chip, as disclosed in U.S. Pat. No. 5,032,897, are expensive to fabricate and do not allow use of existing integrated circuit chip packages.
Another problem arises because conventional electrical circuits often contain several integrated circuit chips mounted in close proximity to one another to obtain faster operating speeds. For example, main memory chips, processor graphics chips, SRAM chips, and cache RAM chips are typically mounted adjacent to one another. However, the closely mounted chips further increase the heat dissipation problems. Conventional cooling methods ineffectively cool the closely mounted chips, particularly because the integrated circuit chip packages have different dimensions and heights, which prevent uniform cooling of the integrated circuits. To prevent overheating, the integrated circuits are operated at lower operating frequencies, reducing the speed of the electrical circuitry.
During the initial power-up of the integrated circuit chips, another problem occurs when the thermoelectric cooler is not yet operating at the desired sub-ambient temperature, or when the temperature of the chip has not stabilized. Operating the integrated circuit chips at high frequencies before uniform cooling of the chip is reached can cause unreliable operation of the integrated circuit chip, resulting in errors during the initial start-up operation of the chip.
A further problem occurs during operation of integrated circuit chips at sub-ambient temperatures. When the chip is operated at low temperatures, thermal expansion stresses are generated between the lower operating temperature chip and the higher operating temperature electrical circuitry, because of the difference in thermal expansion coefficients of the integrated circuit chip and surrounding electrical circuitry. The thermal expansion stresses can cause breakage of the chip, separation of the chip from its mounting, or failure of the electrical leads connecting the chip to the surrounding circuitry.
Thus, there is a need for a cooling system that effectively cools integrated circuit chips so that the circuits can be operated at higher operating frequencies. It is also desirable for the cooling system to cool the integrated circuit, without heating the immediate environment of the circuit. It is further desirable for the cooling system to allow uniform cooling of a cluster of closely mounted integrated circuit packages. It is also desirable for the cooling system to be adaptable for use with existing integrated circuit chip packages.